Under CSA A23.1, hot-weather concreting practice begins when the ambient air temperature is at or above 27 °C, or is forecast to rise above 27 °C during the placing period. Above that, concrete loses slump faster, sets faster, and is more prone to plastic-shrinkage cracking and cold joints. CSA A23.1 Table 14 caps the placing temperature at 30 °C for the under-1-metre sections most residential footings, walls, and slabs fall in (32 °C for sections under 0.3 m).
A drum-mix load that leaves a plant and drives a hot, windy Calgary Chinook day for 45 to 60-plus minutes heats up and burns the CSA two-hour discharge clock faster, and can arrive with its slump gone or needing water added on site, which lowers strength. A volumetric (mobile-mixer) truck batches fresh on arrival, so the concrete is mixed at the site at the right placing temperature and the clock starts when the pour starts. The long hot drive never touches the load.
This is the hot-weather sibling to the cold-weather discharge-temperature article. That piece covers the cold end, the 5 °C trigger and the minimum placing temperatures; this one covers the heat. For the two-hour-clock physics, see Volumetric vs ready mix; we link both rather than re-derive them.
When does “hot weather” officially start?
It’s earlier than most people guess, and it’s a real threshold, not a feeling. Hot weather, per CSA A23.1, is “when the ambient air temperature is at or above 27 °C, or when there is a probability of the temperature rising above 27 °C during the placing period (as forecast by the nearest official meteorological office).” The provision is CSA A23.1, Clause 7.1.1.
(Source: Ready Mixed Concrete Association of Ontario, Hot Weather Concreting Technical Specification, reproducing CSA A23.1; clause number per Dufferin Concrete, Hot Weather Concreting Technical Bulletin.)
This is the mirror image of the cold-weather article’s 5 °C trigger, the other end of the same temperature window. A typical Calgary summer afternoon trips it routinely.
What heat does to a load
The reason 27 °C matters is what happens above it. The CSA-reproduced guidance lists the adverse impacts of placing concrete in hot weather directly. They include:
- “Increased water demand”
- “Accelerated rate of slump loss, often leading to the addition of water at the job site“
- “Faster setting rate, causing more difficulty in handling, consolidating, and finishing, and increasing the risk of cold joints“
- “Higher likelihood of plastic shrinkage and thermal cracking”
- “More challenging control of entrained air content”
And in the hardened concrete: “Reduced strength due to additional water being added on site,” increased drying shrinkage, thermal cracking, and lowered durability.
(Source: RMCAO Hot Weather Concreting, corroborated by Dufferin Concrete.)
Notice two mechanisms in that list line up exactly with a “compromised load”: the slump goes, so water gets added on site, which lowers strength; and the faster set raises the cold-joint risk. Those aren’t vague cautions, they’re the standard’s own description of what heat does.
The placing-temperature ceiling
CSA A23.1 Table 14 sets the maximum temperature concrete can be placed at, and it’s tied to how thick the section is.
| Section thickness | Min placing temp | Max placing temp |
|---|---|---|
| < 0.3 m | 10 °C | 32 °C |
| ≥ 0.3 – < 1 m | 10 °C | 30 °C |
| ≥ 1 – < 2 m | 5 °C | 25 °C |
| ≥ 2 m | 5 °C | 20 °C |
Table 14 also notes, verbatim: “The placing temperature should be kept as close as possible to the suggested minimum temperatures. Higher temperatures result in an increase of mixing water, increased slump loss, and an increase in thermal shrinkage.” For high-performance concrete, “in no case shall the placing temperature exceed 25 °C.”
(Source: RMCAO Hot Weather Concreting and Dufferin Concrete, Table 14.)
Most residential footings, walls, and slabs are under 1 metre thick, so the ceiling that applies to them is 30 °C (or 32 °C for thin sections under 0.3 m). This is the same Table 14 the cold-weather article uses; that piece cites the minimum column (10 °C), this one cites the maximum column (30 °C). Two ends, one table.
A separate US-referenced cap, ACI 305.1, limits maximum fresh-concrete discharge temperature to 35 °C absent supporting evidence. And there’s a strength cost to running hot: as a rule of thumb cited in the Calgary Concrete Master Knowledge Base, for every 1 °C the placement temperature exceeds 25 °C, the 28-day compressive strength drops by roughly 0.5 MPa on a 30 MPa target mix.
(Source: ACI 305.1-14 and the Calgary Concrete Master Knowledge Base, Standards section.) That’s a strength-loss point, hot concrete loses strength, not a claim that any method is stronger.
Why the long drive makes it worse
Heat is one problem; heat plus a long haul is the problem this article is named for. A drum-mix truck batches at the plant, and the concrete starts hydrating and heating immediately. The drive then does two things at once: it spends the discharge clock, and on a hot day it lets the load warm further toward, or past, that 30 °C ceiling.
The standard itself tells you to minimize time-to-discharge in heat. After mixing, “ensure it is discharged promptly without delay.” Its recommended controls include superplasticizers, retarders to extend placing time, scheduling pours “early in the morning or later in the afternoon,” sunshades and windbreaks and fog spray, and chilled water, ice, or liquid nitrogen at the plant.
The plant’s fix for the drive is to chill the concrete before it; on-site batching removes the drive. Hot weather also effectively shrinks the usable window. The national CSA limit is 2 hours, but as a documented example of the principle, projects under Ontario provincial standards require that “when the air temperature exceeds 28 °C and the concrete temperature exceeds 25 °C, the concrete shall be discharged within 1 hour after the introduction of the mixing water.”
(Source: Dufferin Concrete, citing OPSS 1350.)
That’s an Ontario rule, cited only to show that heat compresses the window; the governing Canadian limit is the CSA two-hour clock (Clause 5.2.4.3.1). On a hot Calgary day, a 45 to 60-plus-minute acreage haul can spend a large share of that window before the gate even opens.
Why a Calgary Chinook is the worst case
Calgary doesn’t just get hot; it gets hot and windy and dry, often all at once, and wind is a primary driver of the surface evaporation behind plastic-shrinkage cracking.
ACI 305R sets the quantified trigger: “precautions should be taken when the evaporation rate exceeds 0.2 lb/ft²/hour (1.0 kg/m²/hour),” and the largest drivers are “wind, relative humidity, ambient temperature, and concrete temperature.”
(Source: ACI 305R-20; see also the ACPA Evaporation Calculator.)
A Chinook stacks every one of those drivers in the wrong direction. Calgary sees roughly 30 to 35 chinook days a year, with temperature swings of 20 to 30 °C and sustained winds of 30 to 60 km/h gusting past 100. (Source: Calgary Concrete Master Knowledge Base, climate sections.) Heat, high wind, and dry air together push surface evaporation toward, or past, that plastic-shrinkage threshold faster than heat alone ever would. A fresh slab on a Chinook afternoon is the textbook worst case the evaporation trigger is meant to catch.
For broader Calgary context: the city sees more than 30 °C on about 5 days a year on average, with a record high of 36.7 °C (August 10, 2018), and 25 °C-plus days are common from roughly mid-May to mid-September, routinely tripping the 27 °C hot-weather trigger and pushing thin-section placing temperatures toward the 30 °C ceiling.
The fix: batch on site, skip the hot drive
The cleanest way to keep a hot drive from compromising a load is to remove the drive. A volumetric truck carries the materials separately and batches fresh at the chute, so the concrete is mixed at the site at the right placing temperature and the discharge clock starts when the pour starts, not 45 minutes ago at a heating-up plant.
John Kim, senior research engineer at CTS Cement, makes the materials-side point: volumetric “eliminates the need to factor in unexpected haul times.”
(Source: Equipment World.)
On a long acreage or lease-pad drive on a 28 °C day, a drum load is already partway through its window and warming before the gate opens; on-site batching sidesteps that entirely.
Two things to keep clear. First, on-site batching isn’t stronger than plant concrete, it meets the same ASTM C685 / CSA A23.1 standards; the hot-day advantage is avoiding the haul-and-heat degradation, not strength. Second, hot-weather practice changes placement temperature and timing, not the durability spec: the summer mix is still Type HS or HSb sulphate-resistant with 5 to 7% air for Calgary’s S-2 soils.
One more summer note: Calgary’s hot-pour season overlaps hail and convective-storm season, so fresh-slab protection is a parallel discipline through the same months.
When a plant truck is still the right call on a hot day
Honest triage:
- Short, reliable haul with hot-weather controls in place. If the plant is 15 to 20 minutes out, will dose retarder and supply chilled concrete, and you pour early morning before the peak, a drum delivery can stay within Table 14 comfortably.
- High single-pour volume with a tight finishing crew. A string of drum trucks on a short loop can move large volume fast, with the load chilled at the plant and placed promptly.
- Evening or early-morning placement. Scheduling around the heat, as the standard recommends, can keep a plant load in spec even in summer.
Volumetric is the strongest fix where those break down: the long haul that warms and ages the load, the Chinook afternoon with wind and dry air stacking the evaporation case, the remote site no plant can serve in time.
FAQ
At what temperature does hot-weather concrete practice start in Canada? Hot-weather practice begins at an ambient air temperature of 27 °C or above, or when the temperature is forecast to rise above 27 °C during the placing period, under CSA A23.1, Clause 7.1.1. That’s the mirror of the cold-weather 5 °C trigger.
What is the maximum temperature for placing concrete? CSA A23.1 Table 14 caps placing temperature at 30 °C for sections from 0.3 to under 1 metre, which covers most residential footings, walls, and slabs, and 32 °C for sections under 0.3 m. Thicker sections have lower ceilings, and high-performance concrete must not exceed 25 °C.
Why does a hot day ruin a long-haul concrete load? Above 27 °C, concrete loses slump faster and sets faster, raising plastic-shrinkage and cold-joint risk. A long drive on a hot day spends the CSA two-hour discharge clock while the load warms, so it can arrive with its slump gone, prompting water to be added on site, which the standard notes reduces strength.
How does on-site mixing fix a hot-day pour? A volumetric truck batches fresh at the site, so the concrete is mixed at the right placing temperature and the discharge clock starts when the pour starts rather than back at the plant. The long, hot drive never gets a chance to degrade the load. As CTS Cement’s John Kim puts it, volumetric eliminates the need to factor in unexpected haul times.
Does hot weather shorten the discharge time limit? The governing CSA limit is 2 hours, but heat compresses the usable window in practice. As one documented example, Ontario provincial standards require discharge within 1 hour once air exceeds 28 °C and concrete exceeds 25 °C. The principle holds anywhere: a hot day leaves less usable time.
Why is a Calgary Chinook day especially hard on fresh concrete? Plastic-shrinkage risk rises when surface evaporation exceeds about 1.0 kg/m²/hour, and the biggest drivers are wind, low humidity, and heat. A Chinook stacks all three, heat plus sustained 30 to 60 km/h winds plus dry air, so it pushes evaporation past the threshold faster than heat alone.
Is the summer mix still sulphate-resistant? Yes. Hot-weather practice changes placement temperature and timing, not the durability spec. The summer mix is still Type HS or HSb sulphate-resistant with 5 to 7% entrained air for Calgary’s S-2 soils.
Planning a summer concrete pour in Calgary? Talk to Omega before you book your delivery. We’ll help you determine whether ready mix or on-site volumetric concrete is the better choice for your haul distance, weather conditions, and project schedule—so your concrete arrives ready to perform, not already working against the clock.
